Excellent, looks like it will soon be all sorted. I will just make a bug report next time.

As you say, I had selected Basic with a prefix of /60 before I chose the Config File Override, so yes, I only got the choice of 0 to f. Selecting Basic and /56 and applying it prior to selecting Config File Override allowed me to "Enter a hexadecimal value between 0 and ff here," for the prefix id in the track interface section.